In the current state of the art, standard PET imagers are bulky devices that are placed in dedicated imaging rooms and require the patient to be transported to the imager. These standard imagers have poor spatial resolution that is inadequate for accurate imaging of small organs such as the prostate. In addition, in some clinical situations, there would be an advantage to having a dedicated imager that can be, for example, assisting in the surgery suite where imaging can provide immediate biopsy guidance of (suspected) cancerous lesions, and with cancerous tissue removal both from prostate and from surrounding tissue.
Existing mobile PET imagers do not satisfy the special combined requirements of size, resolution and sensitivity for prostate imaging tests.
A dual-modality prostate PET imager with transrectal ultrasound (TRUS) was described in “Initial Results of a Positron Tomograph for Prostate Imaging”, J. S. Huber et al., IEEE Transactions on Nuclear Science, Volume 53, Issue 5, Part 1, October 2006 Page(s): 2653-2659. TRUS provides anatomical details that can be co-registered with a PET image. This PET imager was constructed from sectors of a standard ECAT HR+ PET with spatial resolution limited to approximately 4-5 mm FWHM. The geometry was closer than standard ring geometry which introduces additional depth of interaction error.
The concept of high-resolution PET imaging in the pelvis region with dual planar detectors has been investigated. For example, see “PET prostate imaging with small planar detectors”, T. G. Turkington et al., T.R.Q Nuclear Science Symposium Conference Record, 2004 IEEEQ Volume 5, Issue, 16-22 Oct. 2004 Page(s): 2806-2809. The scanner consisted of two 20 cm×15 cm (axial) planar detectors made of 3 mm×3 mm×10 mm LGSO scintillator detection elements. The detector heads were mounted on a rotating gantry with adjustable detector radii. Although detection of hot lesions in the pelvis with small dual planar PET detectors was judged to be possible, better characterization of such lesions requires detector orbiting or larger detectors.
A transrectal high resolution (˜1 mm) prostate imager 20 including a PET probe 21 operating in conjunction with a small field of view outside imaging detector 22 and placed by the pelvis region close to the prostate 23, see FIGS. 1 and 2, was proposed by C. Levin in “New Photon Sensor Technologies for PET in Prostate-Specific Imaging Configurations”, and by W. Moses in “Dedicated PET Instrumentation for prostate imaging”, both of which were presented at the Topical Symposium on Advanced Molecular Imaging Techniques in the Detection, Diagnosis, Therapy, and Follow-Up of Prostate Cancer, 6-7 Dec. 2005, Rome, Italy. The prior art probe 21 is behind the prostate 23 and the outside detector 22 is in front of the prostate 23 and serves as a second coincident detector to the probe. The outside detector 22 captures the second coincident 511 keV gamma ray originating from the positron emissions and annihilations in the prostate and in surrounding tissue. The outside detector is placed in a fixed position and includes a limited field of view. In this approach, the limited detector size and limited angular sampling of the imaging procedure does not allow for full scale all-angle 3D tomographic imaging of the prostate region and of surrounding organs.
Other hybrid imaging systems using conventional PET have been proposed, see Sam S. Huh et al., “Investigation of an internal PET probe for prostate imaging”, accepted for publication in Nuclear Instruments and Methods in Physics Research, 2007. A hybrid imaging system 25 with conventional PET, as shown in FIG. 3, includes an external PET ring 26 and combines a conventional PET imager 22 with an add-on transrectal probe 27 to image the prostate 28. A simulation study was performed of a high-resolution imaging probe in coincidence with a conventional external PET scanner. The internal detector provides both high resolution (˜1 mm FWHM) and high efficiency while events recorded by the standard PET provide complete tomographic data for image reconstruction. The concept is still under simulation investigation to estimate the performance in comparison with conventional PET.
A PET system with an insertable probe 29, shown in exploded and assembled form in FIG. 4, as one of the detectors has been proposed by C. Levin, “New Photon Sensor Technologies for PET in Prostate-Specific Imaging Configurations”, presented at the Topical Symposium on QAdvanced Molecular Imaging Techniques in the Detection, Diagnosis, Therapy, and Follow-Up of Prostate Cancer, 6-7 Dec. 2005, Rome, Italy. The second detector would be placed outside the patient on the other side of the prostate in order to capture the second coincident 511 keV gamma ray. Unfortunately, this approach has limited angular sampling of the imaged organ.
There are practical implementation issues related to the use of insertable transrectal probes. They cannot be treated as non-invasive, with the related patient safety issues involving active probes. There are limits to the useful detection volume, probe positioning relative to prostate, and non-uniformity of sensitivity for different parts of prostate. Insertable probes, while being close to parts of the prostate, will not image enough surrounding tissue.
As described above, although some imaging geometries have been proposed for the prostate, there is still a need for a reliable PET imager capable of providing torso-wide imaging of the prostate and surrounding tissues with a wide active field of view and with a sufficiently high resolution for full scale all-angle 3D tomographic images.